(1) Field of the Invention
The present invention generally relates to the use of reactive materials, such as for gettering gases within an evacuated chamber. More particularly, this invention relates to a method of protecting a reactive material during processing so that the reactivity of the material is not degraded for the intended application.
(2) Description of the Related Art
The use of chemically reactive materials to getter gases is a well-known technique for improving the performance of vacuum devices. For example, reactive metals have been employed to reduce the trapped gas pressure of vessels and improve the performance of vacuum tubes, cathode ray tubes, display tubes, flat panel displays, sensors, resonators, micromachined devices, ion accelerators, etc. Evaporated films of barium and zirconium and powders, sputtered films of various reactive metals, and sintered non-evaporable getters (NEG's) in the form of pellets and strips have been employed for such applications. Getter materials often must be “activated,” in which a high temperature vacuum bake is performed to desorb any absorbed gas molecules and to burn off any surface contaminants.
One difficulty with using a reactive material as a getter is that they can be contaminated by handling or through exposure to hot gases during processing steps prior to vacuum sealing. As taught by U.S. Pat. No. 5,191,980 to Boffito et al., an adhesive plastic layer can be applied to a reactive metal film to provide protection during storage and shipping, but must be removed prior to activation and use of the film as a getter material. Therefore, while plastic films of the type taught by Boffito et al. are useful to protect a getter material during shipping, a plastic film cannot protect a getter material during high temperature operation of the device in which film is installed. Furthermore, a plastic film must be removed prior to high temperature processing often required by microelectronic devices, such as many bonded panel and wafer configurations used in the microelectronics industry. During glass frit wafer and panel bonding techniques, glass frit is flowed in air at atmospheric pressure and at temperatures above 300° C. Exposure to these temperatures can degrade the gettering performance of a reactive metal film due to oxidation and gas absorption.
NEG's, commercial examples of which include sintered metal particles and strips available from SAES Getters SpA of Italy, develop an oxide layer during sintering that must be removed by an “activation” step, which involves heating to temperatures of 400° C. or more. Metals such as vanadium and niobium can be added to NEG materials to help reduce this oxide growth, thereby allowing gases to diffuse into the bulk of the material. Nevertheless, activation can require subjecting a NEG material to temperatures that can be incompatible with materials and processes used in the electronic industry.
In view of the above, it would be desirable if contamination, oxidation and gas absorption of a reactive material could be avoided or at least minimized during handling, storage and high temperature processing of a device in which the reactive material is to be used.